text:
Is there employment opportunities of becoming a pilot
I am 18 years old and I want to become a pilot so bad .i got B grades in math and C grades in physics and geography.but my parents don't want me to be one since is hard to get employment of becoming a pilot.So am asking is there any employment opportunities of becoming a pilot

It's as hard as any other job which demands skills. The bad (or good depending on the point of view) thing is that to enter the club you need to have more than 100k ready to spend. If you manage to get into one of the cadet programs by CAE or L3, you will be at the top of the list for recruitment. One personal suggestion: the selection for being a pilot (with no previous experience) relies a lot on soft skills. At 18 humans are still young, get a degree in the meantime to improve your soft skills

Dave is absolutely correct in that there is an unprecedented shortage of crews.  A lot of it, in North America at least, is the effects of the Baby Boom demographic wave passing through the final stages.  Not enough young people learning to fly, too many pilots retiring, more people travelling than ever.
There has never been a employment picture for pilots like this. Never.  That said, it's still a long difficult road but with much higher chances of success than 20 or 30 years ago.
If you are the kind of person that drops what you are doing to watch airplanes fly overhead, anytime anywhere, then you should go for it because you are likely to stick it out.  Make a plan, figure out what to do and go step by step and don't let naysayers discourage you.
Try for whatever state sponsored programs are available; military or civilian.  If that doesn't work and if marks aren't top notch that may be the case, take the longer road.
You'll have to become employable and get a normal job to finance your flying, and to have a back up in place if it all falls apart because you fail or medical out.  DO NOT GET MARRIED during all this or you will be compelled to give it up.
Go to university if you can.  If not, learn a trade that pays well and is not too unpleasant to do.  Develop a baseline income stream that is good enough to live on but with enough surplus to pay for flying.  If you have a non-aviation education/income source and build on that, your parents will be much more supportive and you will be able to cruise along, working and flying, enjoying the adventure.
Use the income stream to start on the road.  Private, building hours, Commercial, Instructor and on and on, over a 5-7 year period or so.  You'll be flying Regional Jets long before you are 30 if you just plod along with a patient outlook toward the final goal.  Critical: make sure you make only friends and no enemies along this road.  Unless you are in the US, it is a very small world.
Personally I'm not a big fan of borrowing 60 or 70000 dollars and taking one of those comprehensive college programs.  You come out with 300 hours and a mountain of debt and the entry level flying jobs pay very poorly.  Become an instructor or fly in the bush for a few years, and you will be a infinitely better pilot than the 300 hr college grad.

+1 for mentioning medical problems.  Get your medical as soon as you start learning to fly so that you'll know where you stand at the outset

By the time you get your Commercial checkout (or ATP) or what ever your local jurisdiction requires you should have no issues finding a job just based on the numbers. There is a growing shortage of pilots and its no secret there are jobs available:

Pilot Shortage Is Real and Getting Worse
The pilot shortage
Cargo Airlines face pilot shortages
A 'Perfect Storm' Pilot Shortage Threatens Global Aviation


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Can anyone identify this project please?
This is a large photo of what looks like a model, with an Armstrong Whitworth stamp on the back.  Any ideas please?

It appears to be a model of the Armstrong Whitworth A. W. 169. There is a good discussion at secretprojects.co.uk where they discuss the offset cockpits for the pilot and radar operator, including why the RO's cockpit is blacked out.

Ma\ny thanks indeed.

source: Stack Exchange

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sort: votes


text:
How can I customize my training to become a flight instructor?
I plan on starting my flight training soon at the age of 41, but I don't want to stop at PPL and simply fly for fun: I want to ultimately be a CFI.
With this goal in mind, my question is what recommendations can my fellow aviation enthusiasts and experts give me in the planning I should do for my aviation education? Is there a specific type of facility to select for training, a progression of obtaining certificates and ratings I should follow, etc.?

I guess the big question I'd ask you first is: "Are you doing this for fun, or are you planning a career change?" -- It doesn't change a lot of the advice I'd give you, but it changes how I'd advise going about the actual training...

I'm voting to close this question because I believe it's too broad for there to be a single good, definitive answer.

I want to make aviation training atleast a part time career if not full time.

Career advice of this kind is off-topic on the entire network.

@AsheeshR I'm not sure this qualifies as "career advice"  - it's not "How can I most easily land a job as a CFI?", but rather "What can I do to optimize my training path to get to this specific goal?". That said I'm also not 100% convinced this sort of question is on-topic for the site myself (probably worth a Meta discussion) - I answered it mainly because I found myself pondering the same thing shortly after I got my private ticket: "What would be the most efficient way for me to get to the CFI rating so I can instruct if I want to?"

How is it in any way different from how do I become a better programmer class of questions? Career advice does not only mean job related advice or job hunting, it covers career growth as well. Unless there is an extremely specific path to follow, this class of questions are a huge can of worms.

@AsheeshR If you feel strongly about it let's continue the discussion on meta. (I personally have no strong feelings either way, either about this specific question or the general class, but I DO feel pretty strongly that trying to have discussions of topicality and site policy here (in comments on the main site) is absolutely the wrong way to handle them...)

First, if you haven't already go check out AOPA's Let's Go Flying site - specifically their page on how to pick a flight school/instructor. Some of the advice I'm going to give you is pretty much ripped right from them.
For my two cents, if you're going for a career move (even a part-time side job type career), you want to optimize your training path - you do not want to be like me where you're doing this whole aviation thing "just for kicks" and can be be a perpetual student pilot taking as long as you want to get each rating!
Generally speaking this means you want to follow the "traditional" path of certificates and ratings in your chosen aircraft category and class. For Airplanes that would be Private Pilot, Instrument Rating, Commercial Pilot, and CFI.
You probably don't want to "detour" to ratings like Sport Pilot (unless you want to become a sport pilot instructor and don't intend to ever become a "regular" CFI, in which case this path lets you skip the instrument and commercial ratings).
The private->Instrument->Commercial->CFI path is "typical" in my experience, and it generally seems to make sense because the skills you use in each phase naturally build on each other and will help you with the more difficult tasks the next rating will demand.
It's probably also worth spending a little time with the Part 61 requirements for each certificate and rating you intend to pick up and planning ahead -- for example, if you can economize by making sure your instrument rating cross-country flight time requirements also satisfy some or all of the requirements for the Commercial rating you can avoid having to do basically the same flying twice.

As far as where you do your training, a lot of that depends on your personal situation (funding, schedule and personality).  The general rule here is "the more you fly the faster the training goes".  I was a weekend warrior for most of my private training, and between weather delays and extended gaps between lessons it took me a LONG time to get that first ticket - the instrument rating may end up taking just as long!
If you're a good student, can take a substantial block of vacation, and have the money to throw at your training an accelerated/immersive program is often the most cost-effective and shortest route to a given certificate or rating: You are effectively becoming a full-time flight student, flying and studying every day, and when you eat, breathe, and sleep aviation you generally pick things up a lot quicker.
As an added advantage these are often Part 141 programs, which reduces the hour requirements for the certificates and ratings (but even if the training is conducted under Part 61 you're still at an advantage because of the immersive nature of the program).
If becoming a full-time student is not an option for you a structured Part 141 program may still be: Some Part 141 schools have "weekend warrior" programs that follow the same syllabus, and have the same benefit of reducing the hour requirements. The drawback is they're not as "immersive", so information may not sink in as quickly and skills will probably take longer to develop - you'll take longer (and probably spend more money) as a weekend student than you would as a full-time student.
The last option is "regular" Part 61 training, which is what most of the folks I know with day jobs they can't escape from do. This is certainly the most flexible option as you're basically training on your own schedule and program. Good instructors will almost universally still be working from some kind of syllabus to keep your training on-track and moving forward, but there's more freedom to jump around and change up the order of lessons, which is nice if you're getting stuck, or bored, or need to really work on a specific maneuver/concept for a while.
The downside is the same as I already mentioned for "weekend warrior" 141 training: the time between lessons really puts a hurting on your mechanical skill development and knowledge retention, and you'll probably wind up taking longer to finish your training.
There is however no reason you couldn't finish up a Part 61 training program right around the bare minimums for each certificate/rating, assuming you fly frequently and don't hit too many personal stumbling blocks along the way.

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Can anyone identify this pusher plane from apparently the 1930s?
I have performed two searches - a photo search and also an FAA NC number search and failed at both to identify this pusher airplane.  My family photo says it was taken by my mother at the Chicago airport in September 1937.  The NC number appears to be NC 15574 and I've tried several variations including dropping the "1" and trying "9" for the "4" but could not get an FAA NC match.  Your assistance in identifying this airplane and anything about it is greatly appreciated.

If this is supposed to be a family photo taken by your mom, why does it have a copyright watermark with your name on it?

My mother passed away at 100 years old two years ago.  She was an avid photographer and aviation enthusiast, which she passed on to me, and she also passed on all of her photography to me, over 30,000 photographs and slides.  I inherited the copyright from her as curator and as you noted her, give her credit for her photography.

That is a Stearman-Hammond Y-S1, registration NC15524, construction  number 309.
The remains of the aircraft are now owned by the Dutch aviation museum  Aviodrome. They are planning to restore one of the four Stearman-Hammonds they have into flyable condition. This project is running since 2007, and is progressing very slowly.

source: Smithsonian National Air and Space Museum

yep, also the number is 15524. Destroyed on 4th nov 1941

@Federico, where did you find that the aircraft was destroyed in 1941?

According to the FAA registration webpage, the S-H Y-1S was deregistered Nov. 4, 1941 and listed as destroyed.      DEREGISTERED AIRCRAFT
Deregistered Aircraft 1 of 1
Aircraft Description
Serial Number 309 Certificate Issue Date 10/25/1941
Manufacturer Name STEARMAN-HAMMOND
Model Y-1S
Year Manufacturer 1937 Cancel Date 11/04/1941
Reason For Cancellation Destroyed
Type Registration Partnership
Aircraft Registration Prior to Deregistration
Name GREEN A B & SABIN GLEN L
Street MUNICIPAL AIRPORT
City BILLINGS
State MONTANA Zip Code 59101
Country UNITED STATES

@FreshBreeze thank you. The FAA registration webpage was down when I checked earlier.

Some other pictures: https://www.google.com/search?q=stearman+hammond&tbm=isch&ved=2ahUKEwi8ktqHx9PuAhVEk54KHXFAACkQ2-cCegQIABAA&oq=stearman+hammond&gs_lcp=CgNpbWcQAzIECAAQHjIECAAQGDoGCAAQBRAeOgYIABAIEB5QhZoFWP25BWDNwgVoAHAAeACAAb0BiAHzCZIBAzAuN5gBAKABAaoBC2d3cy13aXotaW1nwAEB&sclient=img&ei=tKYdYLzoGcSm-gTxgIHIAg&bih=501&biw=1158&client=firefox-b-1-d

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Is the longitudinal extremity of an obstacle limitation surface based on the threshold or the displaced threshold?
In a case where a displaced threshold (DTHR) is present, should the longitudinal extremity of the obstacle limitation surfaces (for example, the conical surfaces) be counted from the threshold (THR) or from the DTHR (case of Moroccan airport)?

Hello Yassine Oubaslam, welcome to aviation.stackexchange.com! I added a tag 'ICAO' to your question as I assume the jurisdiction you are asking about does not deviate from the ICAO recommendations in the context of this question. I you want to have an answer specific for your country, please add that country to the tag-list.

Hello Yassine, thank you for adding the country. I've converted it to a tag (the Morocco tag did not exist yet, congratulations on first Morocco question!). I think the Morocco aviation laws are quite close the ICAO standards and recommendations, so I left the ICAO tags as well. I think Annex 14 describes the obstacle limitation surface, you could have a look there and see whether the DTHR is mentioned in the relevant section.


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What aircraft is used in the movie Plane (2023)?
Does anyone know what type of aircraft was used in the new film Plane staring Gerard Butler?

Can you see a registration/tail number at any point in the movie? If so, it will probably be possible to look up that number and find the type.

Don't recall seeing one

Short answer: It's an MD-80 which can fly at 40k feet and happens to have a glass cockpit.

Complete answer: Inspect the following images from the filming set and trailer...

This is an aircraft from the MD-80 series. This does not seem to be an MD-90 or Boeing 717 since the latter two had visibly different engines.
So case closed? No :-)
Inspect the next two images from the trailer...

Yeah, that's not an MD-80 cockpit; this is what an MD-80 cockpit looks like:

Additionally, the altimeter shows 40,000 feet. MD-80/90 and Boeing 717, all have a service ceiling of 37,000 feet.
But the yoke does look "MD'ish" since the checklist holder does not move with the yoke - a distinctive feature of the MD yokes. Also, the seating layout in the cabin (as seen in the trailer) appeared to be 2-3, which is true for MD-80's.
So it appears that this is indeed an MD-80 - but with several nuances, which is to be expected from Hollywood movies.
Image Sources: First image, Images from the trailer, Real MD-80 cockpit image.

The first cockpit screenshot you show might be from an MD-11 (not sure though, definitely not B717). The second cockpit screenshot is certainly taken from an Airbus (looks like A320/330/340). Typical Hollywood consistency :D

As far as the glass cockpit: From Wikipedia: Later models could be equipped to a higher specification with EFIS displays in place of the traditional analogue instruments, TCAS, windshear detection, etc. An EFIS retrofit to non-EFIS-equipped aircraft is possible. So it seems plausible that there could be a glass cockpit, but I'll take it from those who see it looks like an Airbus that they got the wrong one.

It's more or less standard practice in Hollywood that any time a plane is featured, the inside doesn't at all match the outside.  It's highly unlikely they filmed any of this inside an actual plane.  This is probably a sound stage made to look like a plane.  They know the vast majority of audiences either won't be able to tell or just won't care.

@manassehkatz-Moving2Codidact That image from airliners.net appears to be an MD-82 that is equipped with the Honeywell EFIS dual displays. They replaced electro-mechanical attitude indicators and HSIs that were in the original MD-80.

Comments are not for extended discussion; this conversation has been moved to chat.

After watching the Plane (2023) Trailer,  I am sure it is some variant of the McDonnell Douglas MD-80.

Capt seems extraordinarily stupid, blundering right through the Mother Of All Cells lol.  OTOH, if you've read Fate Of The Hunter by Ernie Gann, they would do just that in their DC-3s and hang on for dear life.

Do those planes still have stairs between the engines?

Rear boarding stairs were an option on these planes. Many were delivered without the rear air-stair. I don’t know which version was more popular.

Thanks Mike for your answer on the stairs.


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In ADS-C-agreement, what is a reporting plan?
ADS-C agreement. A reporting plan which establishes the conditions of ADS-C data reporting (i.e. data required by the air traffic services unit and frequency of ADS-C reports which have to be agreed to prior to using ADS-C in the provision of air traffic services).
I can't understand "reporting plan".
What is a reporting plan?
Is it like a flight plan?
Is it a paper?
Is it an app?

I'm not sure of your terminology, but you appear to be describing what the Global Operational Data Link Document (GOLD) describes as ADS-C contracts. ADS-C contracts specify the type of reports required (periodic or event based) and the data to be included in each type of report.
From the GOLD:

4.5.2 ADS contracts
4.5.2.1 In airspace where procedural separation is being applied, the ATSU should establish the following:
a) ADS periodic contract at an interval appropriate to the airspace
requirements; and
b) ADS event contract for the following events:

Waypoint change event;

Lateral deviation event;

Level range deviation event; and

Vertical rate change event of negative 5000 feet per minute
(descent rate).

The contracts are established by the ATSU and sent to the aircraft after login. The "reporting plan" is that the aircraft will provide the reports as specified by the contracts.

Thank you. for full description.


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How to convert barometric altitude to gps altitude?
In my work I have some challenge with an aircraft altitude values:
I was provided with some data from aircraft flight in my country. The data contains variety sensors data (like lat, long, altitude, out air temperature, pressure,...).
I need a way to do a conversion of barometric altitude to GPS altitude so the conversion would be as much consistent as possible without dependency on environmental parameters like year seasons and  aircraft location (for example equator or +-40 deg lat).

You have to know the local barometric pressure at the time of the indication you are concerned to be able to do the conversion. Without that data, you are stuck.

Hi user64609, welcome to aviation.stackexchange. Where does this data come from? Is it recorded on-board or was it received via Mode-S radar / ADS-B? Does the data contain accurate date and time stamps?

GPS and Barometric altitude are referenced to different datums (geometric vs pressure), so strictly speaking you cannot really convert between the two.
Having said that, GPS Height Above Geoid (as opposed to GPS Height Above Ellipsoid) provides the nearest approximation of Barometric Altitude, assuming the barometric altitude you have is referenced to the local QNH.  If all you have is pressure altitude (referenced to 1013 QNH, as reported by transponder data) then you will have some error associated with the MSL pressure of the day of the flight.
So... if the data you have contains barometric altitude referenced to local QNH, then you can equate this with GPS Height Above Geoid. If you need GPS Height Above Ellipsoid you can convert using the Geoid height corresponding with the location of the measurement. See e.g. https://www.unavco.org/software/geodetic-utilities/geoid-height-calculator/geoid-height-calculator.html

The best way to obtain an accurate GPS height estimate requires a static pressure measurement on board the aircraft and an accurate atmosphere model, valid at the time of measurement.
Pressure is measured on board, but often converted to pressure altitude.
If you have static pressure in your dataset, use that. If you have pressure altitude, convert it to pressure using the standard atmosphere model (see ICAO doc 7488)
Now that we have pressure, we take the atmosphere model valid at the time of measurement. This could be obtained from, for example, NOAA or ECMWF.
Within the atmosphere model, typically a GRIB2 grid file, you interpolate the position, time and pressure level and obtain the geopotental height.
The interpolatation requires some thought. Linear interpolation will yield inaccurate results for geopotential height, as the variation of geopotential height with pressure is non-linear but logarithmic.
Now that we have the geopotential height, we can convert to geometric height, using the formulas from ICAO DOC 7488.
The resulting geometric height is measured above mean sea level (AMSL). GPS output is typically with give as height above WGS84 ellipsoid (HAE). To convert between AMSL and HAE, you can use EGM2008.


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How is lift produced when the aircraft is going down steeply?
If a positive angle of attack (AoA) is needed to produce lift, then how is it created when a plane is in a 30 degree nose low dive?
How does it not stall? At this attitude the AoA has to be negative, right?

Hi Zach Barker, welcome to aviation.stackexchange.com. I have reformulated your question a bit so it is clear to everybody what you mean. Can you check if I have correctly interpreted your question. If not, please use the edit function to improve it.

The angle of attack is the angle between the chord of the wing and the incoming airflow.
Let's look at the case where the nose is 30 degrees down as you mention in your answer. The wing chord is typically mounted at a small angle (angle of incidence) with the fuselage center-line (x-axis of the body frame).

Let's assume the AoI is 2 degrees.
The nose is 30 degrees below the horizon,
the chord line of the wing is pointing (30 - 2 =) 28 degrees below the horizon.
the aircraft is traveling on a path 31 degrees below the horizon.

The angle of attack is the angle between the incoming airflow (from 31° below the horizon) and the wing's chord line (28° below the horizon) is still a positive (31° - 28° = ) 3 degrees.
At 3° AoA, the wing produces lift and will not stall.

An aircraft produces lift in respect to air, so the AoA has to be measured in respect to air. The -30° that you are considering are measured in respect to earth, not air.
This picture (source) depicts this attitude in a simple way:

Let's suppose that the airplane is flying with am horizontal speed of 100 and a vertical speed of -70 (unit of measurement is not important). Here -70 means that the airplane is sinking (dive). Now, these combination of horizontal plus vertical speed gives an airflow with an angle of $\arctan(-70/100)=-35°$ in respect to earth (the blue line in the picture)
The aircraft has an attitude of -30° ("chord line" in the picture) but it is invested by an airflow of -35°. Its AoA is the difference between these two angles: (-30)-(-35)=5°. So, in this particular case, even if the attitude is -30° the AoA is actually +5°.

Because the pitch attitude of the aircraft, relative to the horizon, is irrelevant. The angle that is important is the angle between the wing and the direction (relative to the wing) of the airflow impacting the wing (which is the exact opposite of the direction the aircraft is moving through the air, relative to the air).


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Does RNAV 5 requires certification in Japan?
Through the last question, I learned that the C-130H is an aircraft capable of RNAV 5.
Then my colleagues raised another question.
"According to the US Airforce manual, it is true that RNAV 5 is possible, but is it really possible in our country?
(Of course, it is true that the C-130H was imported from the United States)
Is it okay to perform RNAV 5 without some certifications?"
This is a question of regulation or law in Japan.
Does the RNAV5 need to be certified for onboard equipment or for aircraft when operating in Japan?

Hi Tom, from a comment to an answer to the linked question I gather you are from Japan. I added this info to this question, because that is important information when you want to know about regulation and laws.

Thank you for your help


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How to go about designing a custom airfoil?
I am familiar with the NACA 4-digit and 5-digit airfoil design system where airfoils, but I was wondering if anyone had some good reference material or software for designing a completely custom airfoil.
From what I understand, many new aircraft use completely custom non-NACA airfoils, sometimes several different airfoils over the wingspan. I am curious about how the design process goes from specifications of the airflow (incidence angle, perhaps the sweep of the wing, air speed, density, reynolds number etc) to a new custom airfoil?

You could try Wingmaster. http://download.cnet.com/Wingmaster/3000-6677_4-10201447.html

Wing sweep, incidence, and airfoil geometry will all have to be adjusted to achieve the best performance at a certain air speed, density, and Reynolds number.
The software that egid mentioned is a good place to start. While studying aerospace in college, XFOIL was used for basic airfoil analysis in our classes.
Another program used was AVL. This analyzes the aircraft as a whole and can model other things such as dynamic stability.
I'm glad that XPlane was also mentioned, even though I wouldn't consider it the most scientific tool for testing wing geometry. It is an interesting way to test configurations since it does use the airplane geometry to simulate the performance characteristics.
In order to consider more variables such as sweep, dihedral, and different airfoils over the wingspan, computational fluid dynamics (CFD) software is used. ANSYS Fluent is one example. There are many other packages out there, and anyone doing serious commercial airfoil design will have proprietary software and methods for this.
Of course, even in the days of supercomputers and CFD, wind tunnel testing is still very important. Scale models can be tested to validate the computer models and demonstrate performance.
As far as requirements, they can get very complicated. For aircraft, of course, the primary goal is to provide the lowest drag at cruise conditions. However, many other factors include:

Preventing flutter
Provide room for sufficient supporting structure
Provide room for fuel
Methods to manufacture the shape
Good stall characteristics
No bad aerodynamics with fuselage and engines
Enough lift with flaps for low landing speed
Able to accommodate flaps, slats, spoilers, and ailerons
Tip treatment like winglets/raked tips
Wingspan must fit into available space (airport, hangar, aircraft carrier, etc.)

And probably many more. Of course, the air speed and conditions will also affect the fuselage, empennage (tail) and its control surfaces, and engines, so this will also be taken into account.

XFOIL is a piece of free software that appears to be under at least semi-active development (latest update this year, nothing for two years prior). I've never used it and know nothing about it; this was merely the product of a google search for "airfoil design".
XPlane has a feature called Airfoil Maker that I messed with once. It hurt my brain, but not necessarily because it was bad - it's just complicated stuff. The fun part about XPlane for design is that you also get to design or modify aircraft and fly them to see how the changes affect performance, handling, or other characteristics.

XFLR5 is fantastic sofware for airfoil design. It is basically a graphical user interface for xfoil @egid mentioned.
It is very good for low Reynolds number flows, i.e. quite slow flying larger aircraft or model airplanes. Here is a link to a quick tutorial on the software, and there are many other tutorials you will find on Google.
In the latest versions you can even design whole planes.
Larger companies will make use of computational fluid dynamics (CFD) software, which actually solve the governing equations of fluid dynamics for a selected model (Xfoil and XFLR5 solve other very much simplified equations) Comercial CFD packages are however generally VERY expensive, although OpenFOAM is an open source package that is gaining popularity.
CFD is however a science of it's own and if you don't know what you are doing you are probably going to get useless results.
Regards

For a recent design task involving reflex aerofoils we tried to get some data out of this program from www.mh-aerotools.de. There's both an applet and download, I wasn't able to run the applet but the download is decently extensive.
Your problem however is probably that any decently-sized aircraft manufacturer will have their own proprietary CFD software for the purpose and skilled programmers and aeronautical engineers to go with it.

Almost a decade later... the Add-in called "Airfoil Tools" for Fusion 360 (Mac and Windows) has a ridiculous amount of power, and is free.

... but requires an Autodesk Fusion 360 account which hardly can be called free.

Fusion360 is free for personal use - see https://www.autodesk.com.au/products/fusion-360/personal - so is Airfoil Tools itself - so the entire solution is entirely free.

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text:
Do 121.5 MHz ELTs have to be replaced with 406 MHz models?
There has been a lot discussion about replacing 121.5 MHz emergency locator transmitters (ELTs) with 406 MHz ELTs, since 121.5 MHz ELTs are no longer monitored by satellite.
Are private planes required to have 406 MHz ELTs for operation? Is there a deadline for conversion? Since this is a country-by-country regulation, I am interested in

Canada
U.S.
Mexico
Bahamas
Europe
Australia
(other countries, if information is available.)

I can't speak for the other countries, but at the moment (December 2013), at least in the United States, the answer is no.
The FAA and the FCC have been engaged in a bit of a regulatory battle of wills on this subject, (most recently in March of 2013, and covered by AOPA, EAA, and AvWeb pretty extensively) - Basically the FCC wants us to take our 121.5MHz woop-woop boxes and make them go away, because they're pretty lousy for search-and-rescue, and the FAA is, for the moment, telling the FCC to get stuffed because regulating aircraft is the FAA's job.

Now just because you don't have to replace your ELT doesn't mean you shouldn't -- to be honest a 121.5MHz ELT is not much better than having no ELT at all (COSPAS-SARSAT doesn't monitor 121.5 anymore), so you should consider the type of flying you do, and the likelihood of an incident where the ELT might be useful.
The new 406MHz ELTs are vastly superior to 121.5MHz units for search-and-rescue performance, and if I were running a commercial operation, or flying regularly in an area were search-and-rescue help may be hard to come by if I had an emergency (like say Alaska) I'd fork over the cash for a GPS-enabled 406MHz ELT to replace the ancient 121.5MHz unit in my plane in a cold second.

In Australia aircraft which require the carriage of an ELT are to use a beacon which operates on frequencies 406 and 121.5 MHz. CASA ELT Regulation Changes
The regulation they refer to (CAR 252A) can be viewed in it's current form here. Part four is as follows:

(4)  For an emergency locator transmitter, emergency position indicating
           radio beacon or personal locator beacon to be an eligible ELT, it
           must meet the following requirements:

(a)  it must, if activated, operate simultaneously:

(i)  in the frequency band 406 MHz–406.1 MHz; and
(ii)  on 121.5 MHz;

(b)  it must be registered with the Australian Maritime Safety Authority;
(c)  if it is fitted with a lithium‑sulphur dioxide battery—the battery
                  must be of a type authorised by the FAA in accordance with
                  TSO‑C142 or TSO‑C142a.

From a technical standpoint this is actually a sensible rule - it requires ELTs that COSPAS-SARSAT will detect and locate, and preserves the 121.6MHz signal for airborne search & rescue / direction finding. (Given how much of Australia is "remote" from a search-and-rescue standpoint mandating 406MHz ELTs there also seems quite reasonable, though I don't envy you guys the refit cosys on the older GA fleet!).


text:
What is this strange cutout in Class B of Kansas City International Airport?
To the Northwest of the KMCI Class B airspace, there is a strange cut out in SFC/80 airspace, which I highlighted at this link (and shown in picture below):

highlighted in purple - Image Source: www.skyvector.com
There's no airport there, and no obvious reason for the cut out.
What possible reasons could there be for a strange shape like this?

Note that the circle with the 'R' in it is an airport. The "ELTON (Pvt)" label on the map applies to it. It's the field listed as 'B' in rbp's answer. It does seem odd that the other one isn't charted, though.

it seems like it might be the other way around: you have to file a form to add a private airport to a chart: "To register a private use airport with FAA or make changes to an existing private use airport, complete FAA FAA Form 7480-1 Notice of Landing Area Proposal (PDF) in according to the instructions." https://www.faa.gov/air_traffic/flight_info/aeronav/faq/#q3f

There are in fact two airports in the cutout:

An unmarked field (A in top map) just east of Platte City (see below) that corresponds to the NW limit of the cutout. Note that this edge of the cutout appears to parallel the main runway at MCI.

The private Elton Field (B in top map). In the image below, field is boxed in red and runway in blue.

And there's also the VFR reporting point over north Platte City

Thanks, I didn't realize they'd customize a Bravo space for a private field, much less an unmarked one!

because of the N/S and E/W configuration of the runways, the cutout doesn't interfere with arrivals and departures, and indeed it looks like the NS and EW sections of the cutout parallel the runways; usually cutouts like this are radials and DME arcs off a VOR.

@abelenky I think they also did it so that those two airfields are not in Class B. @ rbp's reason of runway directions probably made it easier to make this cutout decision.

unfortunately for the guy landing at Elton, there's only one way in (south) and one way out (north), without busting Class B

Both these airstrips were there before MCI ever existed. The unmarked one to the East was the VP Engineering at TWA, his Private strip.
Flew his Comanche & Ercoupe to MCI for work before it became a Comnercial field while TWA was still based at MKC. TWA maintenance base at MCI was operational 4 years before the terminal was finished.


text:
4 Basic Forces of flight
I am looking for shortened but precise and understandable definitions for the 4 forces of flight for an essay I have to do (Lift, Weight, Thrust and Drag) or key parts I need to emphasise.
Any help or key tips are appreciated

Is this info not readily available from multiple sources on the internet already?

Hi, welcome to aviation stack exchange. This is a very broad question, and one which could (and does) fill books. It's not particularly well suited to a Question & Answer site. If you have specific things you're unclear about please feel free to post them as separate questions

This NASA link will give you a very nice overview on the matter, especially this page.

I rolled back the edit. Please do not deface your posts.

Hello Nathan.moors2, please do not deface your own posts. If your original question has been answered, then please accept the answer that was most helpful by clicking the checkmark next to that answer.

Weight is the force due to gravity in the direction of the center of the earth (down).
Thrust is the force produced by the propulsion system.  Usually generally pointing 'forward' (and fixed to the body), but sometimes otherwise.
Drag is the aerodynamic force parallel to velocity.  It will always act to reduce the velocity.
There are infinite directions perpendicular to velocity.  The aerodynamic force perpendicular to velocity is decomposed into two components -- lift and sideforce.  Lift is the component of total aerodynamic force perpendicular to velocity and also in the symmetry plane of the aircraft.
Sideforce is perpendicular to velocity and also perpendicular  to the symmetry plane of the aircraft.
In many simple analyses, the sideforce is zero and people forget about it.  In these situations, people will say that lift is the force perpendicular to velocity.

What are you really trying to say in that middle paragraph introducing side force?

Simply that stating "Lift is perpendicular to velocity" is not specific enough when there is non-zero sideforce.  In that case, lift is the part (of the aero force perpendicular to velocity) in the symmetry plane and sideforce is the part perpendicular to the symmetry plane.

But no discussion of the 4 basic forces the OP is asking about ever gets into side force.  I don't disagree with the facts you are presenting, but it seems you are adding unasked for complexity to a model that is intended to be a very simple framework.

And of the infinite directions perpendicular to velocity, what purpose is served by breaking out lift and sideforce?  Is not weight opposing lift more useful?  Sideforce is only relevant and useful when discussing uncoordinated flight.

I always thought that the sideforce you describe is just considered part of lift. There's vertical lift and lateral lift... and, I suppose, longitudinal lift.

@MichaelHall The OP asked for 'precise' definitions.  I felt that you can't precisely define lift without restricting it to the plane of symmetry of the aircraft.  At that point, I felt it was worth introducing sideforce to complete the aerodynamic forces.

Something tells me that was more than was asked for, but I don't know the OP's audience either.  And I honestly don't remember if I've even heard the term "symmetry plane" used before...

A picture is worth a thousand words:

In all seriousness though, these are each plain, (and plane) language terms that have commonly understood definitions.  And if uncertain, they can be readily looked up from multiple sources.  Their usefulness comes not from understanding the words themselves as much as integrating them into the visual model showing how they oppose and balance one another in flight.  This forms the foundation for teaching more advanced concepts.
I would encourage you to either provide and cite a book definition, or explain them in your own words to come across naturally.  It is doubtful that anybody here will be able to offer insight not already published in multiple places elsewhere.
NOTE:  For those unfamiliar, the image posted above is in jest.  The four forces of LIFT, WEIGHT, THRUST & DRAG mentioned in the question are typically applied to a generic airplane picture, and I would bet that a majority of non-aviators would probably be able to accurately figure out where each one goes... (if you are new here, give it a try!)


text:
Itavia flight 870: How accurately can the aircraft crash be reconstructed with current technique?
This question is related to the Itavia flight 870. A passenger airliner Douglas DC-9 that crashed into the Mediterranean in 1980. Surprisingly to me the causes of the crash have never been clearly found. The two most likely causes seem to be a bomb on board or a missile from a military plane.
Given that the relict of the plane has been recovered, I assume that different types of explosion leave different trace on the airplane parts. My question is:
with 2024 technical knowledge should it be possible to discriminate the causes of the crash?

Sad to be said but... welcome to the '80s Italy. The Italian Wikipedia (here the English translation) has a good overview of the whole "story" about this "incident", Tom Clancy couldn't have written a better novel.

"should it be possible to discriminate the causes of the crash?" Do I miss something or only opinions can be provided? Discussing what can be changed in the investigation conclusions looks like discussing the causes before the investigation is closed.

@mins The point of the question is exactly to understand if it is possible to discriminate the two likely causes. If it is a matter of opinions then it means it is not possible. This would indeed be an answer.

So the remains of the crashed DC-9 have been in the ocean for over 40 years?  What might be possible with modern investigative techniques examining a recently crashed aircraft is going to be far better than what can be done with something that's been deteriorating for 40+ years in salt water & ocean currents. Maybe if you get lucky an investigator can distinguish between fragments punching through the skin from the inside out instead of from the outside in, but that's hardly modern science to see that... if the remains even have that level of visible detail left.

@RalphJ: 95% of the airplane lies in a museum. No window exploded outward and the restroom (where supposedly the bomb was located) is still intact.


text:
Private airstrip regulations
Once a private airstrip has been certified, do you have to get faa approval to make changes to the runway such as adding crossings, access etc…?

If it's private it does not need certification, you just need to notify the FAA. See e.g. this question

You don't even need to notify the FAA. Form 7480-1 is optional.

@Dave-CFII Notification is mandatory for the airport described. see below

The only requirement for a private airfield is to notify the FAA using form 7480-1. There are exceptions for otherwise regulated (these would not be private), temporary (less than 30 days), and intermittent (less than 3 days/week for less than one year) VFR use of an site that is not an airport. What you describe is a permament, civil, private use airport so you have to notify 90 days before activating or making a change.

Title 14 Code of Federal Regulations Part 157 requires all persons to notify the FAA at least 90 days
before construction, alteration, activation, deactivation, or change to the status or use of a civil or joint-use
(civil/military) airport. (As used herein, the term “airport” means any Landing or Takeoff Area, e.g. Airport,
Heliport, Vertiport, Gliderport, Seaplane Base, Ultralight Flightpark, or Balloonport.)
Notice is not required for:

An airport subject to conditions of a Federal agreement that requires an approved current airport
layout plan to be on file with the Federal Aviation Administration.
Establishment of a temporary airport at which operations will be conducted under visual flight rule
(VFR) and will be used for less than 30 days with no more than 10 operations per day.
Intermittent use of a site that is not an established airport, which is used for less than one year
and at which flight operations will be conducted only under VFR. (Intermittent use means the use
of the site for no more than 3 days in any one week and for no more than 10 operations per day.)


text:
Who was the third person to fly a powered airplane?
The first person to fly a powered airplane was Orville Wright on December 17, 1903. The second was Wilbur Wright on that same day.
The Wright Flyer was destroyed before anyone else flew it. The Wright Flyer II, I think, never had a pilot besides the two of them. The Wright Flyer III carried Charles Furnas as the first airplane passenger on May 14, 1908.
Who was the third airplane pilot? Was there another person who flew one of the Flyers? Did someone else succeed before that happened?

Welcome, Plutor, to aviation.stackexchange.com. I edited your question to focus on powered flight as unpowered aircraft (gliders) had been flown in the 19th century by Otto Lilienthal and others before the Wright brothers started flying.

To fly a powered airplane or to fly a Wright's airplane? Who flew first a powered airplane is still not clear and most probably will never be.

I'll nominate Alberto Santos-Dumont, who

on 23 October 1906 flew about 60 metres [≈200 ft — ed.] at a height of two to three metres with the fixed-wing 14-bis (also dubbed the Oiseau de Proie—"bird of prey") at the Bagatelle Gamefield in Paris, taking off unassisted by an external launch system.
...
Europeans at the time believed it was the first such achievement. ... The judges had been overcome with emotion and forgot to time and track the flight, and due to this the record was not made official.

(Image from Wikimedia Commons)


text:
What is the validity of an ICAO frozen ATPL?
I would like to  know what is the validity of a frozen Airline Transport Pilot License under ICAO regulations?


text:
What's this part on the inner part of the wing on a Cessna 152 - opposite of the thermometer?

Trying to figure out what this part is.  Sorry for the dark pic, was simming in bad weather.


text:
Do a full 360 on engine failure?
One episode I remember well from my training was a lesson on engine failure. To this day I am still a little uncertain about the takeaway from that lesson, which was by a secondary instructor, not my main instructor.
What happened is that we were cruising along around 3000 feet AGL and he reaches over and closes the throttle, putting the engine on idle: "You have an engine failure and cannot restart it, what are you going to do?"
I scan the landscape in front of me and see a muddy pond with a road next to it. From my glider experience I know I can land the plane in 50-100 feet on the bank of that pond and it will be an easy landout because the road is right there. So, I make for the pond.
The instructor says, "No, no, do a 360". So, I do a 360 and lo and behold there is a small 2500-foot private airstrip right behind me. What a coincidence. I line up and deadstick onto the airstrip. He opens the throttle before I touch down and the lesson is over.
So, in that situation I had a place I knew I could land OR I could do a 360 and look for an even better place at the cost of losing altitude. Doing a 1-minute turn at -500 feet/minute, the altitude penalty for doing the 360 is probably going to be between 500 and 800 feet (out of my 3000). Is it the right policy to do the 360 or go for the first landable location you see as I had done originally?

check your map to see if a airstrip is nearby.

I suspect the instructor's point was not to recommend doing a 360 to look for a landing site, it was that you should have known the airstrip was there because you were aware of your position.

If you need to check your map to look for an airstrip, you're already lost.

@Pondlife The lesson was not on cross country technique and had nothing to do with foreknowledge. I have updated my question to clarify this.

Even if you're unfamiliar with the area, instructors want you to constantly look out for potential landing spots and your description suggests that you more or less overflew the airstrip. I think he probably expected you to see it and make a mental note; many (most?) student pilots have had exactly the same experience and felt rather silly when the instructor pointed out the airstrip right under the wing. I know I did!

I believe it is also important whether, after completing the 360, you would be still able to make the originally chosen spot.

Tyler this is a conversation you should be having with your instructor, not with people on the internet where you have no idea what their qualifications are. If you are unhappy with your instructor's response have a chat with your club CFI.

Maybe your CFI's point was that a better option may be behind you, and you should look around a little bit. I would not agree however with a 360° turn, a 90° turn would be sufficient to check behind you and get a surrounding look (before turning, look 90° then turn 90° opposite of direction you looked). Turns are expensive as far as altitude goes when gliding. If you saw something behind you, then continue the turn.

Unconditionally, NO.
The principle is that you should prefer worse result you can certainly achieve over better result you can possibly achieve. You had a landing spot where you could make a survivable landing. That is good enough. Look for anything else only if:

You actually noticed your flew over a small airstrip. I would expect the instructor to hint that they actually expected you to notice it though if he expected it¹.
The original landing spot is close enough that you can make a 360 and still make that spot if you find nothing else.

But by all means, you must never ever give up a spot for survivable emergency landing spot unless you see a spot that provides safer landing at is at least as certainly reachable.

¹ I do think it was the original point of the lesson, but I can imagine the instructor themselves didn't really get it when they learned it and therefore reproduced it poorly.

There is no indication in the question that doing a 360 would have made the original spot unreachable.

Point #2 is the closest thing I have received to an answer. Thank you very much. In the situation in question I had the extra time to do the 360 because the pond was close by, so by that criteria I should have done the 360. Of course, then I would be cutting it closer if I had found nothing better, so I would have had less margin for error.

@DJClayworth, but there is no indication it would either.

The lesson here is not about the 360 but about locational awareness (the airstrip being there was no coincidence). One of the best quotes I have ever heard from an old pilot is

"A flight is not from point A to point B as much as its a series of
  hops from one potential landing spot to another"
-- An Old Pilot

The take away is that your instructor was checking to see if you knew where you were and thus knew there was an airfield behind you (and subsequently out of sight). In your case there is nothing incorrect about picking a field and heading for it. If you have sufficient altitude and no suitable field is found a 360 is not a bad choice to check for a field.
On another note, a pond is not a suitable field for landing for a variety of reasons. First off you were (presumably) in a fixed gear plane. While belly landing a retract on water can be preformed safely, belly landing a fixed gear plane on water can be dangerous since the wheels will create a lot of drag when they hit the water. Mud can also be dangerous as you wont really roll on it as much as you will sink hard into it, and potentially nose over.

I learned to fly in mountainous parts of Washington State, where viable landing spots are often few-and-far-between.  Keeping track of your nearest potential "out" was very important.   I now fly in the flat parts of Kansas.... every field, every road, every "everything" is a potential "out", and I now give very little thought to potential landing spots; they're everywhere.

This is incorrect. The instructor flew me around to make sure I did not know where I was. As for your ideas about landing on water and mud, you have obviously done neither. Are you even aware that Alaskan bush pilots land on muddy river banks every day?

Landing on muddy banks with the proper tires and off strip landing skills is more than possible and I never claimed to have done either, doing so in a tricycle gear trainer like a 172 or Cherokee can be very dangerous. Even if you were lost you should have used VFR reference points to identify your position and deduce where the safe strip was.

As you wrote in your post, "lo and behold there is a small 2500-foot private airstrip right behind me."  To me, that implies you passed right over it not 1 minute earlier.  Whether or not you knew your location on the chart, you should have seen and remembered an airstrip that you just passed over, and your first reaction should have been to turn towards the closest airstrip.

Is doing a 360 a normal part of engine out procedures?
First, you should not even really be asking such a question here.
For emergency procedures, you probably want to check the FAA Handbooks, or other training materials.  Your POH will detail procedures, and your instructor is a reliable source of info.
Given that you did ask here:  No, doing a 360 is not an advisable emergency maneuver, especially at 3,000 AGL.  (You may be able to make an argument that the altitude loss is worth the value at much higher altitudes.  But 3,000 is pretty low).

Why did my instructor ask me to do a 360?
He was encouraging you to locate an ideal landing spot that you already should have been aware of.  Although he kept you busy, situational awareness, both with respect to the ground and other VFR aircraft, is a non-negotiable item.  You cannot skip it when you're "busy".  Spotting a 2,500 airstrip from 3,000 AGL should've been a no-brainer.
I'm confident that he was not instructing you in "normal" procedures when he told you to do a 360.  He probably should have made that clear, both at that time and during the after-flight de-brief.

But I had a good landing spot picked out
I don't think you'll find many people who like your description of "a muddy pond with a road next to it" as a good landing spot, despite what Alaskan Bush Pilots are capable of.   (Note that most bush pilots will make several passes over a landing spot before finally touching down.  You don't have that luxury with an engine out.)
Can you imagine an NTSB report that says the pilot died after chosing a muddy pond when a perfect airstrip was less than 2 miles away?
You did not have a good landing spot picked out, and you may recall that over-confidence in one's own abilities is a major risk factor contributing to accidents.  Please dwell on that for some time.

What the instructor said in the debrief was "always go fully around before selecting a landing location". That was the debrief. So, your assumption is wrong.

You originally wrote, "Is it the right policy to do the 360 or go for the first landable location you see as I had done originally?"   Now that I've answered that exact question, you still don't like the answer??   I also simply don't believe he said that.   Ask him if you should go fully around at 1000 AGL?  With a full airstrip already in front of you? In rising terrain?  In all those situations, obviously, you should not go fully around.

Yeah, and I don't believe you would notice a 2500 x 100 grass strip cut out of trees even if you flew right by it at 3000 AGL. So, I wonder who is right? You are claiming you would notice something like Asplund field just casually flying by it? Not likely. The strip in my case was even smaller than Asplund with tall trees literally right on the sides of the 100-wide foot runway. No way you are seeing that unless you are looking for it and even then you have to be right on top of it to see it.


text:
What are the differences between intertank valve and crossfeed valve?
Regarding the fuel system in an aircraft, I read that there are two valves: the intertank valve and the crossfeed valve.
They are both motor driven butterfly type valve which function is to allow fuel from the left and right hoppers to combine.
What are the differences between them?

In layman's terms, at least on a Gulfstream G450/G550, the cross-feed system allows the engines to be fed from either the left tank, the right tank, or both tanks (one to each engine.) the cross-feed pipes feed into the fuel hoppers, not directly into the other tank. While you can use the cross-feed system to fix a fuel imbalance, doing so would be via using the fuel (in the engines) not actually transferring it from tank to tank.
The intertank system is a valve and pipe system between the tanks themselves, allowing a fuel imbalance to be fixed by initiating/allowing the transfer of fuel from tank to tank directly.
My sources were:
G450/G550 Fuel System
G450/G550 Fuel Unbalanced

Sources look good to me; independent verification is pretty much always preferred over appeal to authority on SE anyway :-)


text:
What could cause uneven fuel levels in a Cessna 172?
I went for a 1.5 hour flight in a 1977 Cessna 172N, with several takeoffs and landings. I was alone. When I left, I had 18 gallons in each tank. When I arrived back, I had 15 gallons in the left tank, and 10 in the right tank.
Why was there such a disparity? Is this at all dangerous? Why did the right tank have less?

semi clog in the fuel line, pump not working optimally

Well, on the bright side: 7.3 gallons per hour is a pretty nice fuel burn :-)

I've flown a number of Cessna 172s (N and S models) the last few years.  Every single one of them burned noticeably more from one tank than the other.  Consistently too, it was always the same tank on a particular airplane.  I too am very curious as to the reason, since the fuel system is so simple and it looks like it should stay even.

@rachet freak  No fuel pumps in a C-172N.  Gravity feed only.

I think Skip pretty much just answered the question. Gravity feed isn't consistent, so one side may flow better than the other. Or maybe you made more left turns. I wouldn't worry that much about that kind of fuel burn. You can report it to the mechanics and have them take a look. But unless there's a BIG difference (like one wing doesn't seem to be burning fuel at all), then the most they're likely to do is clean or replace some filters.

I was just going to say, aviation's all about left turns, so I wouldn't have expected anything less from a gravity feed system with inlets at the wing roots.

@KeithS -- as long as the turns were coordinated, it seems to me that your explanation doesn't work.

@Shawn-- see above comment.  PS-- what do you mean by "Gravity feed isn't consistent"?  Is one wing somehow "feeling" stronger gravity than the other?

Our school SOP tells us to start the engine with fuel on LEFT, then do the after start checklist, then switch to RIGHT before taxiing to run up area where we switch to BOTH before the run up. Depending on how much the engine ran with the selector on the LEFT and RIGHT position, it’s expected to have uneven fuel burn overall.

Consulting your handy POH (or one for a 172N that I Googled up) you'll find the fuel system diagram looks like the one below (click to enlarge - most high-wing gravity-fed fuel systems are similar).

So there are a number of ways you could wind up with uneven fuel burn with this plumbing.
The most common ones are:

(Uncoordinated) Turns
If you're flying lots of left-hand traffic patterns (or you're making lots of left turns for some other reason) that means your left wing is frequently lower, and gravity favors pulling fuel from the right tank. (Note that it doesn't have to be a turn - prolonged slips can also cause uneven fuel flow.)
If I had to take a guess, I'd say this is probably your culprit. Were you making a lot of left turns?
Pressure
The fuel vent tube on the left tank faces into the wind - this pressurizes the left tank first (and the right tank through the breather tube connecting the two) - it's a little extra encouragement for fuel to leave the tanks and flow into the engine. In some aircraft this leads to the left tank draining faster (because it's at a slightly higher air pressure than the right tank) - this can become particularly pronounced if the breather tube between the two tanks is blocked.

Various blockages
A partial blockage in either of the tank fuel lines, the fuel selector valve, the vent, or the breather line between the two tanks could theoretically cause uneven fuel burn (the vent/breather blockages being a special case of "pressure" from (2) above).
Pilot Error
It's possible to mis-position the fuel selector in such a way that you're getting adequate fuel flow to run the engine, but one tank is more restricted than the other.
This is unlikely if the aircraft is properly maintained (the fuel selector should have nice, identifiable detents that it clicks into, letting you know it's in position).

For more detail on causes for uneven fuel burn (and tips for troubleshooting it with your mechanic) check out Cessna Pilot's Association Technote 003.

Is uneven fuel burn dangerous? Maybe.
It's certainly something you should be aware of (part of why you want your fuel gauges to be at least somewhat accurate is so you can tell if your fuel burn is uneven in flight and correct it by switching to the fuller tank briefly).
Uneven fuel burn can be an annoyance (a 5 gallon difference is about 30lbs - potentially enough to notice on the controls), but it can also be a safety hazard: If allowed to go uncorrected you could theoretically get yourself into a situation where the right fuel tank is empty, and in a steep enough left turn you may not have enough pressure from gravity to get fuel from the left tank into the engine.
In an extreme case your right tank may be empty and your left tank may be so low as to unport the fuel pickup tube in a turn, potentially leading to attitude-induced fuel starvation ("The noisy part of the plane gets quiet until you level the wings").

Thanks for the detailed and interesting reply. I looked at my GPS tracks, and made 5 right-hand patterns and 3 left-hand patterns. I didn’t do any other significant turns. So that makes your most likely explanation less likely... time to file a squawk.

@Peter An uneven ramp is sometimes a culprit too (same principle: fuel flows across through the breather line or the valve) - I assume you checked your fuel right after the flight though, and that you'd probably notice if the plane wasn't level. It sounds like you definitely had fuel flow from both tanks (those 3 gallons went somewhere) but if you can't figure out why it was so uneven it's definitely worth having a mechanic look at. Even if they say "no trouble found" at least you'll know there's not a gremlin waiting to bite you on a future flight :)

I fail to see how (coordinated) turns affect your fuel balance.

In a coordinated turn, the resulting gravitational and centrifugal force is always orthogonal to the wings and therefore cannot cause any imbalance in the fuel system. Only uncoordinated manoeuvres can cause any imbalance and one normally aims to fly coordinated.

Indeed as Jan and DeltaLima pointed out if the ball is superglued between the lines you should never have uneven fuel flow as a result of turns. It's uncoordinated turns (either briefly when entering the turn over multiple turns, or flying in a slip for an extended period of time) that muck up the fuel flow.  I'll clarify that in the answer.

If this was a fuel-injected 172, the answer is easy - the excess fuel return line only goes to one tank. No such luck!

@Egid The 172N is carbureted, not injected.

Right. Which is why I'm out of luck.

You guys should check one of the airplanes at my Flight School after a Student Pilot has come back from their Solo Cross Country flight. You could hear one of the two tanks begging the pilot to fly coordinated.

Does Cessna have any non return valves to prevent fuel to go from one tank to another when selector is on BOTH? Otherwise fuel should be always level after some time.

I think this answer as it currently stands still does not clearly convey the point that only uncoordinated turns could cause a fuel imbalance.

I asked my instructor and this was his answer!
The Nefarious Skyhawk Gas Gremlin !
Fact: When a high wing plane (such as a Cessna 172 or a Cessna 150) is flown on a training flight with the fuel selector in the "Both" position (C150 fuel valves are always on the "both" position - even when "off") with a primary flight student at the controls, the plane will almost always land with more fuel in the right tank than in the left (but the very same plane may occasionally land with more fuel in the left tank with a slightly more experienced student, or - eventually - with equally balanced fuel tanks).
"Why is this?" you ask.  "How can this be?" you demand to know!  I ask all my students this, but I try not to give them the answer.  The answer is completely obvious once they have flown for a while.  Once they understand the most fundamental basics of flight, they will be able to tell me the secret of "The Nefarious Skyhawk Gas Gremlin".  (But once in a while a CFI is compelled to point out the obvious).
WHAT IT IS NOT:
It is not because of a design flaw in the Cessna.
It is not because of an obstruction in the fuel line or the vent (while that is possible, this rare obstruction problem is not germane to the nearly universal "Gas Gremlin" phenomenon).
It is not because of turns and banks during flight.
An aircraft can do steep turns all day long; loops, wing-overs, and barrel rolls, ad infinitum, with the fuel selector in the "both" position, and never have a fuel imbalance.  Yet a simple training flight will usually result in more fuel in the right tank than the left.
WHAT IS THE CAUSE?
Hint, the answer is related to the "native call of the CFI in his/her natural environment".
The cause is revealed, not by the trusty aircraft mechanic, nor by the psychic aviation consultant, but by the most simple indicator found in all airplanes: The slip/skid ball.  The force that causes the ball to be pulled to the right side of the curved glass tube during takeoff, climb, and power-on stall practice, is the very same force that is pulling the fuel from the left tank into the right wing tank.
This is also the origin of the "native call of the CFI in his/her natural environment" ...  "RIGHT RUDDER!  RIGHT RUDDER!  RIGHT RUDDER!  ... STEP ON THE BALL !!!"
Note: this unbalanced fuel condition will also be caused when even the most experienced pilot is practicing side-slips and forward slips.  But this is OK, and elicits only a silent smile from the CFI.
JT Rairigh, CFII, MEI, Helicopter, A&P/IA

An amusing way of reiterating point #1 in the heavily upvoted and accepted answer by voretaq7♦.

I have an f model 172 and as with most all skyhawks I have the fuel imbalance problem as well. I found that in the Cessna Service manual, the fuel vent has very specific measurements it is to be set at with only 1/32" + or - tolerances. My 1965 172 shows it to be 4 5/8" down and 1 3/16" behind the strut. I can tell you from experience it makes a notable difference if it is not positioned per the specs. When set correctly the difference is minimized to 2 gallons and always less in the right tank!
While there is allot to be said about coordinated flight involved as part of the problem, I have always been very diligent to have the ball centered and found it didn't make any difference in the imbalance. But the vent position sure does.

This is a small discrepancy in a tank that is quite difficult to measure accurately: it's wide, short, 2 meters in the air, the filler hole is at one end and it's on a contraption that is rather sensitive to the angle of the pavement.
If you have (nearly) ZERO burn from one tank, you have a reason to see your mechanic and probe the fuel lines for a blockage.
The weight difference will not affect your flight. If one tank is empty and the other nearly full you may bank slightly to that side, but Cessnas can easily handle 800 pounds hanging on the wing strut.

I care because it’s a potential safety issue or potentially indicates something wrong. I want to understand the plane more.

I applaud your curiosity but you need to understand that fuel imbalance is endemic to 172s.  One reason is that the fuel vent hangs below the wing in the slipstream and if it isn't positioned perfectly it can partially pressurize one tank, diverting fuel to the other.  However, this is not really a safety of flight problem.  If the gauges are showing a severe imbalance in fuel remaining, fly on the fuller tank only for a while.  Yes discuss this with the mechanic but don't be surprised if he shrugs and goes back about his business.

Paul's answer may have come off a little callous, but it really doesn't deserve the downvotes. And he's right. I will also agree with Skip again. This level of burn difference in a 172 during a training flight really isn't that much of a concern. If it makes the pilot comfortable, report it to Mx (just as anything wonky with the plane should be reported). But don't be surprised if the response is a shrug. 15 gals / 2 gals would raise eyebrows. 15/10 isn't really that much of a problem in this aircraft. Especially during a training flight.

Airplanes are complex machines that never work "perfectly". An important part of training is learning what to pay attention to and what can be dismissed. Notice, ask someone who knows (like a mechanic) and file their reply for future reference. But don't pester maintenance with every tiny detail or they will ignore you when you say the engine runs rough (and the next guy loses power on takeoff). In this case, we clearly have fuel flow from both tanks, a burn rate within normal limits, and a quick look at the fuel system diagram shows how it can easily move back and forth. Mystery solved.

I get that a fuel imbalance in a Cessna may be 'normal' and dismissed as not a problem. I am sorry, however, but "Why would you care?" ... it is the fuel system, which affects the whole, as @voretaq7 said so eloquently, "the noisy part of the plane gets quiet..."

Read it as "Why would you care about a small difference....", not "Why would you care about the fuel system". Look at the position of the filler caps in the diagram, look at the stick you use to measure the fuel. Possible causes of a 30% imbalance should be clear.


text:
Geographical locations of ARINC and SITA ground station
Is there an up-to-date list of ARINC and SITA ground stations?
The German Bundesaufsichtamt für Flugsicherung has published a PDF leaflet which lists various ACARS frequencies, including those for ground stations such as

136.825 MHz VDL Mode 2 GND
136.875 MHz VDL Mode 2 GND
136.925 MHz

Now I'm wondering where those ground stations are located.

Why would you need to know that?

It lists those frequencies that are reserved. It does not say they are in actual use.

There is the plain old ACARS and the FAA Next Gen (VDL2). An overview.


text:
Can I use unleaded fuel in PA-23 Piper Aztec?
Minimum octane to PA-23-250 is 91 octane as mentioned in the POH.
Is there any option to add 95 unleaded fuel to my Aztec?

I am not sure which 95 unleaded fuel you are asking about. Is it 95 unleaded Avgas, or 95 unleaded Automotive fuel? Also, are you asking because you want to use unleaded fuel, or are you looking for an alternative to Avgas?

If you are asking about 95 unleaded automotive fuel(Mogas), then No, you can not use 95 unleaded in your Aztec because  there is no STC available anywhere.
The POH for the PA-23-250 Aztec specifies a minimum of 91/96 octane. 91/96 was a military spec fuel and was never really used in General Aviation. Due to the unavailability of 91/96, you will find that 100/130 or 100LL was generally used instead. In 1991 a Swedish company started producing 91/96UL (unleaded), but I am not sure about it’s availability.  I believe it’s available in France, Germany, UK, and Sweden.
There are three different ways to measure Octane Rating. RON, MON, and AKI which is actually (RON+MON)/2.
UK and EU auto gas use RON
USA and Canada auto gas use AKI
Avgas uses MON
MON and is rated much lower than RON. 95 octane RON is equivalent to 91 AKI, which is equivalent to 85 octane MON. This roughly equates to 80/87 Avgas.
The low equivalent octane ratings of Mogas might be why there is no Mogas STC available for the Piper PA-23-250 Aztec. I don’t believe there is an Mogas STC for any fuel injected aircraft that normally requires 100/130 or 100L.
If you run 91 octane AKI Mogas(North America) or 95 octane RON Mogas(EU), you might get away with it for awhile, but I wouldn’t risk it because it is only equivalent to 85 octane MON Avgas.
One risk is possible damage to your engines. The other risk is the possibility your insurance company may not cover any accident claim you may have.

Should the AKI formula be (RON+MON)/2 (i.e. average) instead? Otherwise it doesn't add up, literally.

Yes, you are correct. I edited my answer. Thanks.

The 91 octane requirement just means you can't use 80/87 avgas, and require, in the old days, 100 octane avgas (green) or nowadays 100 Low Lead avgas (blue).  Or any other avgas grade of 91 octane or better, had it existed.
You can use alcohol free unleaded automotive fuel also, of 91 octane or better, if you have the Petersen mogas STC. So the answer is yes, if you incorporate the STC (which is paperwork and placards), and in general the engines will be way happier on unleaded autofuel than leaded avgas (subject to any operating limitations in the STC).
One downside is your plane will now stink like a car, instead of an airplane, but otherwise, the engines will thank you, because except for a benefit related to exhaust valve seat lubrication effects, the lead is bad news.  You also may want to stick with avgas if the airplane's going to sit for extended periods, say, 6 months at a time or more, with fuel in it, since avgas is much more stable.

Thanks - I have a mental image of all the planes on the hard stand, clustering away from the one that stinks like a car.

I use car gas in my plane and I hate the smell.  You know how smells trigger memories better than anything?  When I smell avgas, which is rarely, it transports me back to 1975.

There is no Mogas STC for the PA-23-250 Aztec. There probably never will be due to octane requirements and also Vapor lock problems.

Ah thanks.  Must have been vapour lock problems that couldn't be overcome.  Octane is not a problem even for 100 octane engines unless they are turbocharged and have a high comp ratio.  There is an STC for the supercharged R-985, which runs at 36" of MP at max power.  The air service I flew for in the early 90s, the owner switched to mogas for his DHC Beaver, fixed his rear plug fouling headaches.

The DHC-2 Beaver runs on 80/87 so Mogas is fine. The Aztec needs 91/96 minimum, so Mogas doesn’t have a high enough Octane rating. I don’t think there are many 91/96 or 100/130 engines that have a Mogas STC. There is a $10,000 Mogas STC for some  100/130 engines, but it requires the installation of an water-methanol anti-detonant injection (ADI) system.

Some 100 octane engines run fine on 94 octane mogas, like the O-360.  My old glider club had 1000s of hours on O-360 powered (STCd) Super Cubs running premium unleaded.

The question regards the use of unleaded fuel, and the presumed use of 95 octane unleaded fuel. Unleaded 100UL aviation fuel is available that can be used without restriction in aircraft piston engines. In this answer, here, the following was noted regarding unleaded 100UL aviation fuel:

Within a broad perspective, the overall most important issue in purchasing a plane generally seems to involve the cost of operation. The dilemma that was mentioned should be viewed from the perspective of G100UL aviation fuel, details of which can be viewed here. In July of 2021 the FAA approved Supplemental Type Certificates (STCs) authorizing use of G100UL high-octane unleaded avgas in aircraft piston engines, without modification. At least this is something to consider, in the foreseeable future piston-engine aircraft are not likely to be rendered unserviceable due to the unavailability of leaded avgas, or the need to modify engines to use unleaded G100UL fuel.

That's very interesting.  I thought that project was still ongoing.  Can you actually find that fuel anywhere now?

In the U.S., PA-23-250 with the following engine models are approved by FAA to use UL94 from Swift Fuel:

O-540-A3D5
O-540-A1D5
O-540-A1B5
IO-540-C4B5
IO-540-C1B5

https://www.swiftfuelsavgas.com/stc/Piper/PA-23-250/any/any/any


text:
What does the isolation valve do in the landing gear extension/retraction system?
The isolation valve is an integral part of the landing gear extension/retraction systems on most larger aircraft.
What is its funtion?

Which aircraft are you talking about?

Why is this question closed? Someone who knows what is asked, cannot post the answer now.

How can I reopen the question? Can someone help with this.

The question was closed by 5 users. It is now in the re-open stack, 3 votes for re-opening have been registered, and 1 to keep closed, 2 more re-open votes would be required. Only high reputation users can vote to close or re-open. I personally can see enough detail in the question to provide an answer

The function of an isolation valve in a landing gear hydraulic system, is to isolate a critical part of the hydraulic system upon pressure loss in all redundant supplies.
For instance for the B737, from this site:

If both system A and B lose pressure, the accumulator isolation valve closes at 1900psi and you are just left with residual hydraulic pressure and the pre-charge. The gauge will indicate approx 3000psi and should provide 6 full applications of brake power through the normal brake lines (so full antiskid is available) As the brakes are applied the residual pressure reduces until it reaches 1000psi at which point you will have no more braking available.

As a general answer: the purpose of an isolation valve is to separate two (or more) parts of a hydraulic system from each other. The separation is necessary to prevent a failure of one part of the system affecting the rest of the system. For example, a leak in landing gear actuator system might bleed the entire system - rendering the whole hydraulic system inoperable.
An isolation valve may also be used to prevent system operation if certain conditions are not fulfilled. Say, if airspeed is above a predetermined limit, the isolation valve would remain closed preventing the operation of the gear.


text:
How can an aircraft travel faster than speed of sound without breaking the sound barrier?
An excerpt from the BBC article here https://www.bbc.co.uk/news/uk-england-london-51433720
"Despite travelling faster than the speed of sound the plane would not have broken the sonic barrier as it was helped along by fast-moving air."
Could someone please elaborate on how that is possible?

Ground speed vs air speed...

@jcaron, I see, learnt something there. The article doesn't mention anything.

The airplane never achieved a supersonic airspeed per se. It was a flying a normal crew speeds in a very strong tailwind, which yielded a ground speed which would’ve been greater than the speed of sound had it been achieved in still air.

It was a British Airways flight and their callsign is Speedbird for a reason :-)

The sound barrier is a function of airspeed and air density. If you have a strong enough tailwind, your ground speed would be above what most lay people would consider the speed of sound without breaking the actual sound barrier.
Aircraft (or any object moving through the air alone) are governed by airspeed. Ground speed is only relevant when traveling on the ground or transitioning to that state (approach to landing).

They do not make that differentiation or even a mention of groundspeed / airspeed in the article.

@SamerTufail -  That’s the problem with mass media. They are either uninformed, or believe the right answer too complicated to be digestible to the uninformed. They are still in the business of entertainment after all. The news gets aviation information wrong all the time due to their misinterpretation of technical terms as keywords and sound bites. But, those keywords and sound bites sell. They buy our attention.

Absolutely. I knew ground speed / airspeed but then I read this could not add it up, it didn't light up for me and posted a question. Now I feel bad.

Why feel bad? There are no stupid questions. Just stupid people who don’t ask questions. Like the mass media.  Someone once told me that knowing the answers is not as important as knowing what questions to ask.

Yup, I will probably leave this here. Some other mass media victim may find it useful.

"The sound barrier is a function of airspeed and air density." Actually, speed of sound in air only depends on temperature.

The speed of sound is measured in relation to the medium an object (or sounds waves) are moving.
The actual speed is dependant on the tempreture and some other factors of the medium.
Let's take for example the the information here :
From the EngineeringToolbox
We can see that in dry air, 20 celsius, the speed of sound is about 340 m/s.
In adtition, if we measure the aircraft's velocity with relation to the ground, we have to take in account the velocity of the wind.
In the example noted in the question, if we have lets say an aircraft travelling at a ground speed of 380 m/s, but it has a tailwind of 80 m/s, it means that by the ground track- it was supersonic if it would have traveled in relation to the ground. Meaning it does travel faster than the the speed of sound, if the sounds moves with no tailwind. But- in relation to the actual air (enclosed system), it travels in a velocity of 300 m/s, which is about 0.88 mach.
And that means- that a sound wave, in the same air mass- launched from the same origin with a base speed of 80 m/s (the tailwind)- would travel faster than the aircraft.


text:
Can I revalidate my EASA PPL outside the EU in a non-EASA aircraft?
I'm trying to re-validate my EASA PPL licence, registered with the Hungarian CAA. I asked the CAA but I did not get a clear answer.
I meet the criteria for re-validation based on experience, but it also requires minimally one hour refresher training with and EASA (FI) or (CFI). I can not travel to the EU due to COVID measures.
Can I fly outside of EU with an EASA FI for refreshing training on a non-EASA registered aircraft?

You'll need to trawl through https://www.easa.europa.eu/ if nobody can answer directly, but it might also help to specify where abouts in the world you currently are. I know I've certainly heard of EASA instructors operating in  the US for pretty much exactly your requirement.


text:
How does a flap load relief system work?
I recently read about flap load relief systems in some aircraft that attempt to prevent flap overspeeds by automatically retracting flaps under certain conditions.

Are these systems common on all large aircraft or are they specific to just a few?
How do they work (i.e. under what conditions will they retract flaps)?
Do they prevent the possibility of overspeeding flaps altogether, or do they just make it less likely to happen?

To the best of my remembrance there was no flap load relief system on 747-100s and -200s. Also, I can't find any reference to such in the documentation I still have.

Nor is there such a system on the ERJ-170/190 (to add a currently produced transport category aircraft to this discussion).

I'm not sure how common flap load relief systems are - I know the Airbus A330 has them and I believe all the Airbus A3xx series do. I've also found at least one post that seems to imply it's common on most "large commercial" aircraft, which I assume encompasses most transport category aircraft.
On the Airbus the system is based on airspeed - When the maximum flap extension speed (VFE) is exceeded by a certain threshold the ship's computer automatically retracts the flaps.
My understanding is the retraction is staged, so if you've got full flaps selected and exceed VFE for that flap setting the system takes out one notch -- presumably it would continue doing so if you're still exceeding the operating limitations of the flap system until it reaches a point where it's convinced you're not going to damage the airframe.
When the overspeed condition is corrected (the speed has reduced to below Vfe by a set threshold) the flaps are returned to the selected position.

To add on to this old question, here's the load relief schedule for a 777-200ER.


text:
Why don't both black boxes hold the same data?
As I understand, there are two black boxes on-board an aircraft. One black box, the Cockpit Voice Recorder holds the cockpit conversations and the other, the Flight Data Recorder holds essential flight parameters.

But why are the two black boxes holding separate data?
Why don't both black boxes hold copies of both the FDR and the CVR data for extra redundancy in case the other box goes missing or is completely damaged by the crash?
Are there any technical reasons for why this isn't possible or hasn't been attempted yet?
Is there any benefit of having the CVR and FDR in separate boxes?

Black Box(s) is a misnomer, you have correctly mentioned their actual names.

Why are the two black boxes holding separate data?

They have different purposes.
Flight data recorder: This is an electronic device employed to record any instructions sent to any electronic systems on an aircraft. It is a device used to record specific aircraft performance parameters.
Cockpit voice recorder: This device is used to record the audio environment in the flight deck of an aircraft for the purpose of investigation of accidents and incidents.

Why don't black boxes hold copies of both the FDR and the CVR data for
  extra redundancy?

Since they record different data, they are different. They are built different.
Differences:
FDR

Time recorded: 25 hour continuous
Number of parameters: 5 - 300+

CVR

Time recorded: 30 min continuous, 2 hours for solid state digital units
Number of channels: 4

Similarities:

Impact tolerance: 3400 Gs /6.5ms
Fire resistance: 1100 deg C /30 min
Water pressure resistance: submerged 20,000 ft
Underwater locator beacon: 37.5 KHz
Battery: 6yr shelf life, 30 day operation
Both recorders are installed in the most crash survivable part of the aircraft, usually the tail section.
Each recorder is equipped with an Underwater Locator Beacon (ULB) or pinger to assist in locating in the event of an over water accident.

Are there any technical reasons for why this isn't possible or hasn't
  been attempted yet?

It has certainly been attempted.

Is there any benefit of having the CVR and FDR in separate boxes?

Probably the reason is that different teams investigate the data obtained from them as they have to recreate the scenario before the accident.
Regulatory Details

CVR: Due to the highly sensitive nature of the verbal communications inside the cockpit, Congress has required that the Safety Board not release any part of a CVR tape recording. Because of this sensitivity, a high degree of security is provided for the CVR tape and its transcript. The content and timing of release of the written transcript are strictly regulated.
FDR: Newly manufactured aircraft must monitor at least twenty eight important parameters such as time, altitude, airspeed, heading, and aircraft attitude. In addition, some FDRs can record the status of more than 300 other in-flight characteristics that can aid in the investigation. The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms.

I think that all of the data is highly sensitive and there is a high degree of security surrounding it, not just the CVR's!

@Farhan, I meant why hasn't the concept of both black boxes with the same data been attempted yet. Not just the one black box :)

@shortstheory You mean one box which records ALL conversation and ALL data?

@Farhan ah yes. BOTH boxes which carry all the conversations and all the data. Like, both boxes have copies of all the CVR/FDR data.

@shortstheory IMO, redundancies are added only when they are needed. Black boxes are not easily breakable and are almost always found.

There are a couple of reasons that I can think of off of the top of my head:

There are different regulations covering when each of the boxes is required.  Some aircraft only require a CVR while others require both.  With them in different boxes, they can be installed in whatever combination is necessary.
They require connections to different systems in the aircraft, and requiring them both to connect to all systems would result in quite a bit of additional wiring.  It would also expose each box to a potential catastrophic electrical event (voltage spike) from any of the connected systems, potentially damaging both boxes at the same time.
There is already some added redundancy by them being in different boxes.  Even if one fails or is is badly damaged, there would probably be data available on the other one anyway.

As Farhan pointed out, there are already some units capable of recording all data at once, but since the regulations only require one unit there is no added redundancy.  For aircraft that are required to have both a CVR amd a FDR it could be a great option!

@Lnafziger with todays technology you could stream both sets of data to both boxes and store it on both...

@ratchetfreak Perhaps on some of them, but there are still a lot of old analog recorders out there too!

Similar to Lnafziger, here is what I can think of:

Protecting two recorders in one box would be harder than protecting one recorder. The smaller the container, the easier it is to crashproof it.
Adding a second recorder in each box adds complexity, which is not desirable in a system like this.
Wiring up all of the data to both recorders would add complexity to the airplane, which is also not desirable.

The number of cases where your suggestion would actually benefit investigation is not very many. In crashes that damage the recorders too much, making the recorders larger and more complex would probably not help. There have been many cases where the recorders didn't work properly or at all, so giving more functions to each box makes this more likely to occur more often.
Here is a list of unrecovered flight recorders. Considering the time it spans, there are not many, and in some cases, neither recorder was recovered.
I'll see if I can find any references for the cases where they were recovered but contained no useful data.

I know of the flight that crashed on asian soil and law enforcement let the plane burn down however the fire exceeded the limits of the blackboxes

In my opinion, I think that whatever is present in one black box also will often tell the other half of the story as well.
For instance, if the airspeed indicators are unreliable, the CVR will pick it up by the pilot's talking, even if the FDR may be lost or destroyed, which will point investigators in the right direction how to continue the investigation.
In other words, I'd imagine there's a bit of indirect redundancy by two devices.
However, many aircraft have a quick access recorder which although not armoured will give much the same stuff as a FDR. If I'm not wrong, Flight Data Recorders are mostly non-mechanical devices and are tested annually, so they are kept reliable.

Ideally, you are right, you can infer some information about one from the other. However, sometimes the crew fails to make any comments about things, which is difficult without the FDR. And even if you have the FDR, the crew response is often critical, which can be difficult without the CVR.

Indeed there is at least one case where the Flight Data Recorder has been damaged/destroyed and investigators reconstructed some idea of at least engine power settings based on what could be heard on the CVR. (I can't recall which incident it was off hand.)

The reasons for TWO boxes are most likely that they came up with the idea for a cockpit voice recorder first, and later decided to add another one for data.
The magnetic tape and other formats first used took up a lot of space and to put both in one box would have been a big heavy box the size of two put together.  These are about 20 pounds each, a combined box would have been around 40 pounds.
Today, with digital chips replacing the big bulky tape machines, they can be made smaller and lighter and they indeed do make recording of both cockpit voice and data contained in the same box, such as this one...
http://cdn.l3commercialaviation.com/media/1530/mkt067-ap_fa5001_1-17rev1.pdf
And they are also considering have two per plane, one in the tail section as is done now, and the other near the front cockpit, so if one is damaged, info could still be contained possibly from the second unit which would also contain both voice and data.
They are constantly considering all options, from floating boxes, to transmitting live data to satellites as well during flight.